A stylish style restyling awesome Tree Style Tab addon on Firefox, e.g. adding a background image XD

restyle_tree_style_tab.user.css

/* author: 
 *   frantic1048 (https://github.com/frantic1048)
 * description:
 *   a simple stylish style costmizing key style of Tree Style Tab addon for Firefox
 *   Tree Style Tab: https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/
 * usage:
 *   create a blank stylish style, paste this file into editor, click save ~
 * comment:
 *   modify the rules to fit your flavor :P
 */

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

/* whole widget */
.tabbrowser-strip[treestyletab-mode="vertical"] {
  background-image: url("<background image URL as you wish>") !important;
  background-position: bottom left !important;
  background-size: contain !important;
  background-repeat: no-repeat, !important;
  background-attachment: scroll !important;
  background-color: transparent !important;
}
#main-window:-moz-window-inactive 
  .tabbrowser-strip:not([treestyletab-style~="aero"])[treestyletab-mode="vertical"] {
    /* inactive styles */
}

/* tabs */
.tabbrowser-tabs[treestyletab-mode="vertical"]
  .tabbrowser-tab[visuallyselected="true"] {
    background:  rgba(227, 45, 70, 0.7) !important;
    border: none !important;
    box-shadow: none !important;
}
#main-window:-moz-window-inactive
  .tabbrowser-tabs[treestyletab-mode="vertical"]
  .tabbrowser-tab[visuallyselected="true"] {
    /* inactive style */
    background:  rgba(227, 45, 70, 0.7) !important;
    border: none !important;
    box-shadow: none !important;
}

/* tab label */
.tabbrowser-tabs[treestyletab-mode="vertical"]
  .tab-text {
    font-size: 11px;
    font-weight: normal !important;
    color: #000000 !important;
    text-align: start;
    text-shadow: none !important;
    margin-bottom: 1px;
}
.tabbrowser-tabs[treestyletab-mode="vertical"]
  .tabbrowser-tab[visuallyselected="true"] .tab-text {
    /* inactive style */
    font-weight: bold !important;
    color: #fff !important;
    text-shadow: none !important;
}